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Nitrogen study looks at sources, pathways

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The MPCA conducted a study of nitrogen in surface waters so that we can better understand the nitrogen conditions in Minnesota’s surface waters, along with the sources, pathways, trends and potential ways to reduce nitrogen in waters.

About the study

The study was a collaborative effort led by Minnesota Pollution Control Agency, with assistance from the University of Minnesota and the U.S. Geological Survey. The report team used more than 50,000 water samples collected at 700 stream sites and used 35 years of monitoring data and findings from 300 published studies.

Precipitation amounts have a pronounced effect on nitrate loads. During a dry year, loads may drop by 49% compared to an average year, however during a wet year, overall loads may increase by 51%.

Nitrate concentrations and loads are high throughout much of southern Minnesota, resulting largely from leaching through large parts of intensively cropped soils and into underlying tile drains and groundwater.

Cropland sources account for an estimated 89 to 95% of the nitrate load in the Minnesota, Missouri, and Cedar Rivers, and Lower Mississippi River basins.

Tile drainage pathwayIn tiled cropland, most of the rainwater that ends up in surface water (ditches, streams) flows through tile drainage. This water can be high in nitrate, but it is also potentially easier to control.

Groundwater pathwayIn cropland without tile drainage, most rainwater flows through the ground to get to surface waters. As it travels through the earth, some of the nitrate is removed, resulting in less nitrate reaching our streams and rivers. However, there are fewer options of controlling this kind of nitrate pollution once it moves below the crop roots.

Where does the nitrate go?

Groundwater nitrate can take from hours to decades to reach surface waters.

The Minnesota River adds twice as much nitrate to the Mississippi River as the combined loads from the Upper Mississippi and St. Croix Rivers.

On average, 158 million pounds of nitrate leaves Minnesota per year in the Mississippi River — 75% comes from Minnesota watersheds.

Nitrate concentrations have steadily increased in the Mississippi River since the mid-1970s.

Nitrate loads leaving Minnesota via the Mississippi River contribute to the oxygen-depleted “dead zone” in the Gulf of Mexico (currently estimated to be the size of Massachusetts). The dead zone cannot support aquatic life, affecting commercial and recreational fishing and the overall health of the Gulf.

How do we reduce the nitrate going into surface waters?

Tactics for reducing cropland nitrate going into surface waters fall into three categories:

Nitrate fertilizer efficiency has improved during the past two decades. While further refinements in fertilizer rates and application timing can be expected to reduce nitrate loads by roughly 13% statewide, additional and more costly practices will also be needed to make further reductions and meet downstream needs. Statewide reductions of more than 30% are not realistic with current practices.

To see progress, nitrate leaching reductions are needed across large parts of southern Minnesota, particularly on tile-drained fields and row crops over thin or sandy soils. Only collective incremental changes by many over broad acreages will result in significant nitrogen reductions to downstream waters.

What’s next?

A state-level nutrient reduction strategy is being developed to address Minnesota’s contribution to the Gulf of Mexico’s hypoxia issue. Minnesota contributes the sixth highest nitrogen load to the Gulf. The strategy will identify how further progress can be made to reduce nitrate and phosphorus entering both in-state and downstream waters.

The MPCA is also working with the U.S. Environmental Protection Agency and other states to evaluate the effect nitrate has on aquatic life in order to develop and adopt toxicity standards.